kvclient/kvcoord: remove stale TODO in Step

We currently have a TODO in Step to add an assertion that stepping is
only done on the RootTxn which references a now-closed issue. I did
a little bit archeology, and in bd255e5
we consciously removed the assertion like this since stepping on
a LeafTxn should be no-op, should just work, and is actually needed
behavior for internal executors when they run on remote nodes as part of
DistSQL plan. (In connExecutor we currently unconditionally Step the txn
on the main query path.) Thus I don't think we want to implement the
TODO and should just remove it.

Release note: None

Author: yuzefovich

pkg/kv/kvclient/kvcoord/txn_coord_sender.go

@@ -1525,11 +1525,6 @@ func (tc *TxnCoordSender) TestingCloneTxn() *roachpb.Transaction {
 
 // Step is part of the TxnSender interface.
 func (tc *TxnCoordSender) Step(ctx context.Context, allowReadTimestampStep bool) error {
-	// TODO(nvanbenschoten): it should be possible to make this assertion, but
-	// the API is currently misused by the connExecutor. See #86162.
-	// if tc.typ != kv.RootTxn {
-	//	return errors.AssertionFailedf("cannot step in non-root txn")
-	// }
 	tc.mu.Lock()
 	defer tc.mu.Unlock()
 	if allowReadTimestampStep && tc.shouldStepReadTimestampLocked() {